Spherical Copper Fine Powder and Process for Producing the Same

ABSTRACT

Provided is spherical copper fine powder in which the average grain size of copper fine powder is 0.05 μm or more and 0.25 μm or less. Additionally provided is a method of producing spherical copper fine powder including the steps of preparing a slurry by adding cuprous oxide to an aqueous medium containing an additive of natural resin, polysaccharide or a derivative thereof, adding 5 to 50% of an acid aqueous solution to the slurry at a time within 15 minutes, and thereby performing disproportionation. The process enables speedy, efficient and stable production of metallic copper particles controlled in particle shape or particle size, particularly copper fine powder having small particles in size.

TECHNICAL FIELD

The present invention relates to spherical metal copper particles having a controlled grain shape or grain size, and in particular to a method of producing spherical copper fine powder which can achieve speedy, efficient and stable production of metallic copper particles controlled in particle shape or particle size, as well as to the spherical copper fine powder obtained thereby.

BACKGROUND ART

As methods of producing copper powder, the electrolytic method and the atomization method have been used conventionally. The copper powder prepared based on these methods can be favorably used in powder metallurgy of oil retaining bearings and electrical brushes, but finer particles with controlled grain size and grain shape are being demanded for use as conductive fillers such as paint, paste and resin in which the demands thereof are expected to increase in the near future.

The methods of producing such fine metal copper particles that suit for the foregoing usage are:

-   -   (1) hydrogen pressure reduction method of copper salt aqueous         solution;     -   (2) chemical additive reduction method of copper salt aqueous         solution; and     -   (3) thermal decomposition method of organic copper salt.

However, there are problems such as equipment costs and operating costs being expensive, and there are drawbacks in the yield is inferior upon controlling the particles to be of a prescribed grain shape and grain size, surface oxidation occurs easily, or the chemical costs are expensive, so these are no satisfactory methods.

In light of the above, it has been known that the method of reacting cuprous oxide particles and acid enables the favorable control of the grain shape and grain size of the metal copper particles to be created, and by additionally managing the reactive conditions such as the pH, temperature, and average retention time, it is possible to adjust the prescribed grain shape and grain size, and produce high purity metal copper fine particles.

In addition, it has also become known that it is possible to obtain chain-shaped agglomerated/bonded powder by selecting the reactive condition (for instance, refer to Patent Document 1).

Patent Document 1 was published in 1985, and was extremely high technology in terms copper powder production at that time.

The outline of the technology above is as follows:

-   1) a method of collecting metal copper particles by reacting cuprous     oxide particles and acid to generate a copper salt aqueous solution     and metal copper particles, and performing solid-liquid separation,     including the steps of continuously pouring a diluted acid solution     into a reaction tank at a flow rate of obtaining a prescribed     average retention time corresponding to the target grain size of the     metal copper particles to be produced, adding cuprous oxide     particles at an adding rate of maintaining the pH of the reaction     tank at a prescribed value and causing a reaction at a liquid     temperature of 50° C. or less, discharging a slurry of the metal     copper particles to be created at a rate that corresponds with the     flow rate of the solution, collecting the metal copper particles via     a solid-liquid separation means from the discharged slurry of the     metal copper particles slurry, and thereby producing metal copper     particles having a controlled grain size, and -   2) a method of collecting metal copper particles by reacting cuprous     oxide particles and acid to generate a copper salt aqueous solution     and metal copper particles, and performing solid-liquid separation,     wherein reaction is performed while maintaining the liquid     temperature capable of obtaining the prescribed grain shape and     grain size.

Nevertheless, in recent years, even finer and more uniform copper powder is being demanded, and technology for producing such copper powder quickly is also being sought. In light of the above, the present inventors proposed a method of producing copper fine powder in which the disproportionation start temperature is set to 10° C. or less upon producing copper fine powder by performing acid-based disproportionation to cuprous oxide in an aqueous solution containing an additive of natural resin, polysaccharide or a derivative thereof (refer to Patent Document 2).

This method enables the speedy production of fine copper fine powder, and is extremely effective. Nevertheless, the average grain size of this copper fine powder is at a level of 0.5 μm to 3.0 μm, and the present inventors were searching for a method for even finer copper powder.

-   [Patent Document 1] Japanese Patent Laid-Open Publication No.     S60-33304 -   [Patent Document 2] Japanese Patent Laid-Open Publication No.     2005-256012

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a method of producing spherical copper fine powder which provides speedy, efficient and stable production of metallic copper particles controlled in particle shape or particle size, particularly copper fine powder having smaller particle sizes, as well as to provide the spherical copper fine powder obtained thereby.

The present invention provides:

-   1) Spherical copper fine powder, wherein the average grain size of     copper fine powder is 0.05 μm or more and 0.25 μm or less; and -   2) The spherical copper fine powder according to paragraph 1) above,     wherein the specific surface area (BET) of copper fine powder is 2.5     m²/g or more and 15.0 m²/g or less.

Here, the term “spherical” means a shape in which the ratio of the short diameter and long diameter of the individual copper particles is 150% or less, and in particular 120% or less. Thus, a shape in which the ratio of the short diameter and long diameter exceeds 150% is of a flat shape, and is not referred to as “spherical.”

With the present invention, even in cases where flat copper fine powder gets mixed in, the amount thereof is 20% or less of the overall amount, preferably 10% or less, and more preferably 5% or less. In reality, desirably, such flat copper fine powder is not contained.

The present invention additionally provides:

-   3) A method of producing spherical copper fine powder, including the     steps of preparing slurry by adding cuprous oxide to an aqueous     medium containing an additive of natural resin, polysaccharide or a     derivative thereof, adding 5 to 50% of an acid aqueous solution to     the slurry at a time within 15 minutes, and thereby performing     disproportionation.

As the additive, natural rubber or gelatin may be used. As specific examples of such an additive, pine resin, gelatin, glue, carboxymethylcellulose (CMC), starch, dextrin, gum arabic, casein and the like are effective.

Though the slurry concentration of the cuprous oxide is suitably 500 g/L or less, the process is usually carried out at 300 g/L or less. This slurry concentration can be suitably selected without any particular limitation. If the slurry concentration of the cuprous oxide is made to be extremely low, since the reaction will not progress, it will just increase costs.

The molar ratio (predetermined number of acids/number of moles of slurry) is desirably 1.00 to 2.00 upon implementing the process. There will be no problem with the reaction so long as the molar ratio is equivalent (1.0) or higher. The effect will not change even if acid is added excessively. Contrarily, if the acid concentration is too high, the calorific value upon adding acid to the cuprous oxide slurry will increase, the temperature of the reaction system will increase, which is expected to be disadvantageous in obtaining finer powder, and in terms of cost, disadvantageous as well.

Meanwhile, if the acid concentration is low, the reaction rate will consequently deteriorate, and this will be disadvantageous in obtaining finer powder. In light of the above, it is desirable to set the molar ratio (predetermined number of acids/number of moles of slurry) to 1.00 to 2.00.

Desirably, the disproportionation start temperature is set to 10° C. or less in producing copper fine powder by performing acid-based disproportionation in an aqueous medium. This is effective in forming copper fine powder with fine particles.

Moreover, it is extremely important that this acid aqueous solution be added at a time, namely, at a time within 15 minutes. It is thereby possible to obtain spherical copper fine powder having an average grain size of 0.25 μm or less. The disproportionation based on the speedy addition of acid aqueous solution is able to achieve fine spherical copper powder. The reason why this collective addition in a short time is effective in producing copper fine powder is not necessarily clear.

Nevertheless, this short-period disproportionation is considered effective on the growth of copper particles. Thus, collective addition in a short time is effective in achieving finer powder. Preferably, the adding time of the acid aqueous solution is short, that is, 3 minutes or less, and more preferably 1 minute or less.

The present invention further provides:

-   4) The method of producing spherical copper fine powder according to     paragraph 3) above, further including the steps of performing     solid-liquid separation and water cleaning to the copper fine powder     slurry obtained after the disproportionation, additionally     performing alkali solution-based reduction treatment thereto, and     repeating the solid-liquid separation and water cleaning of the     obtained fine powder slurry to obtain copper powder. This alkali     solution-based reduction treatment yields the effect of achieving a     uniform chemical composition of the copper particles by reducing the     cuprous oxide that has not yet reacted with the oxide remaining in     the obtained copper fine powder. -   5) The method of producing spherical copper fine powder according to     paragraph 3) or paragraph 4) above, wherein acid-based acidification     treatment is performed during the course of repeating the     solid-liquid separation and water cleaning of the fine powder     slurry. This acid-based acidification treatment is able to enhance     effect of the rust prevention in performing rust treatment. -   6) The method of producing spherical copper fine powder according to     any one of paragraphs 3) to 5) above, further including the steps of     filtering the copper powder after the final water cleaning     treatment, and additionally performing vacuum drying thereto in     order to obtain the copper powder; -   7) The method of producing spherical copper fine powder according to     any one of paragraphs 3) to 6) above, wherein the average grain size     of the copper fine powder is 0.05 μm or more and 0.25 μm or less;     and -   8) The method of producing spherical copper fine powder according to     any one of paragraphs 3) to 7) above, wherein the specific surface     area (BET) of the copper fine powder is 2.5 m²/g or more and 15.0     m²/g or less.

The method of producing copper fine powder according to the present invention yields superior effects of achieving a spherical grain shape and arbitrarily controlling the grain size, and enabling speedy, efficient and stable production of copper fine powder having smaller particle sizes.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] A diagram showing the outline of the production flow of the spherical copper fine powder

[FIG. 2] An FE-SEM photograph of the spherical copper fine powder

BEST MODE FOR CARRYING OUT THE INVENTION

The cuprous oxide particles may be produced with a publicly known method such as from a copper salt aqueous solution via cuprous chloride. Specifically, since there is no direct relationship between the grain size of the cuprous oxide particles to be used and the grain size of the metal copper particles obtained by the method of the present invention, coarse cuprous oxide particles may be used.

Sulfuric acid is commonly used for acid, but nitric acid, phosphoric acid, or acetic acid may be used. It is not necessary to specify the type of acid. In case sulfuric acid is used, disproportionation is to generate a copper sulfate aqueous solution and metal copper particles based on the following reaction formula.

Cu₂O+H₂SO₄=Cu↓+CuSO₄+H₂O

If the additive ratio of acid to the cuprous oxide is increased, then the pH of the reaction system will decrease, and the pH will increase in the opposite case. Thus, the pH can be controlled based on the additive ratio of acid or cuprous oxide.

The pH is maintained to be 2.5 or less, and desirably in the vicinity of 1.0 in order to avoid the generation of precipitation of impurities during the reaction and to promptly advance the reaction without leaving any residual cuprous oxide.

Upon producing copper fine powder based on disproportionation of cuprous oxide, acid-based disproportionation is performed in an aqueous medium including an additive (protective colloid) of natural resin, polysaccharide or a derivative thereof. This is a major characteristic of this invention.

This additive (protective colloid) works to inhibit the growth of particles, and also works to reduce the frequency that the particles come in contact with each other. Accordingly, this is effective in producing fine particles.

As the additive, natural rubber or gelatin may be used. As specific examples of such an additive, pine resin, gelatin, glue, carboxymethylcellulose (CMC), starch, dextrin, gum arabic, casein and the like are effective. In particular, if glue is used, it is possible to achieve fine powder having an average grain size of 0.25 μm or less, and yield an agglomeration inhibiting effect.

The liquid temperature during the reaction is set to 30° C. or less, and preferably to 10° C. or less upon producing metal copper fine particles. If the liquid temperature exceeds 30° C., there is tendency of the metal copper fine particles becoming agglomerated and bonded with each other. In particular, it is desirable to set the disproportionation start temperature to 10° C. or less in order to seek finer powder. As a result of lowering the reaction temperature, the growth of particles can be effectively inhibited, and even finer powder can be obtained.

With temperature of 10° C. or less being maintained till the end of the reaction, there will be a better effect. It is also possible to set the reaction temperature to exceed 30° C. In the foregoing case, a special grain shape can be obtained by leveraging the tendency of the metal copper particles becoming agglomerated and bonded with each other. As described above, the grain shape and grain size of the metal copper particles to be generated can be controlled based on the reaction temperature. The present invention covers this kind of temperature control.

Moreover, upon producing copper fine powder by performing acid-based disproportionation of cuprous oxide in the present invention, it is extremely important that this acid aqueous solution be added at a time. Specifically, the acid aqueous solution needs to be added at a time within 15 minutes, preferably within 3 minutes and more preferably within 1 minute. It is thereby possible to obtain spherical copper fine powder having an average grain size of 0.25 μm or less.

The disproportionation based on the speedy addition of acid aqueous solution is able to achieve fine spherical copper powder. Thus, by speeding up the adding rate of acid, nucleation will prevail over the growth of particles, and a finer copper powder can be obtained.

This short-period disproportionation is considered to have an effect of inhibiting the growth of copper particles. Thus, collective addition in a short time is essential in achieving finer powder.

The average grain size of the present invention is desirably a small value, but if the value is smaller than the average grain size (D₅₀), the actual value of D₁₀ becomes 0.06 μm, and D_(min) as the minimum value of grain size distribution will become even smaller. However, with the disproportionation process, which is a wet reaction, since 0.05 μm is lower limit of production, the average grain size is set to 0.05 μm.

Since D_(min) as the minimum value of the grain size distribution is even smaller, much finer copper powder will be included. In addition, with the disproportionation process, which is a wet reaction, since 0.05 μm is estimated to be lower limit of production, the average grain size is set to 0.05 μm.

Meanwhile, the smaller the average grain size becomes, the larger the specific surface area tends to be, but they are not necessarily proportional. In addition, the measured value and the theoretical value of the specific surface area are different.

When presuming that the copper fine powder is a true spherical shape, the true density of copper is 8.93 g/cm³, and calculating the volume, surface area, and mass from the specific surface area with the average grain size (D₅₀) as the diameter, D₅₀=0.05 μm, and the theoretical specific surface area will be 13.44 m²/g.

Nevertheless, with respect to the relationship between the average grain size (D₅₀) and the specific surface area, the tendency is: the smaller the average grain size becomes, the less the difference between the theoretical value and the measured value becomes. This is considered to be because the surface state (irregularities on the outermost surface) will affect the specific surface area when the average grain size is large, while in case the average grain size becomes small, the influence of the size itself becomes greater than the surface state and the difference between the theoretical value and the measured value becomes less.

In summary, if the lower limit of D₅₀ is set to 0.05 μm, it is anticipated that the upper limit of the specific surface area will become approximately 15.0 m²/g. Thus, the upper limit of the BET specific surface area was set to 15.0 m²/g.

The ultrafine spherical copper powder obtained as described above could become agglomerated in the air or liquid. Nevertheless, the agglomerate itself can be dispersed once again with a means such as applying ultrasonic waves in the aqueous solution. It should be understood that this is based on the premise that the initial particles are spherical copper fine powder having an average grain size of 0.25 μm or less. This is because spherical fine copper powder cannot be obtained by attempting to achieve finer powder by way of pulverization.

When performing batch-type reaction, acid may be added to the slurry of cuprous oxide particles, or contrarily cuprous oxide particles or a slurry of cuprous oxide particles may be added to the acid solution.

In all cases, the obtained metal copper particles are of high purity and have abundant surface activity. Accordingly, appropriate rust prevention treatment is performed to the metal copper particles obtained from the solid-liquid separation, and the metal copper particles are subsequently dried. FIG. 1 shows the outline of the production flow of the spherical copper fine powder.

The spherical copper fine powder is produced through the processes as shown in FIG. 1: dissolving additive→obtaining slurry (process of adding cuprous oxide into an aqueous medium containing an additive to form a slurry)→disproportionation (addition of acid aqueous solution)→cleaning→rust prevention→filtering→drying→pulverizing→sorting.

Examples

The present invention is now explained in detail with reference to the Examples. These Examples are merely illustrative, and the present invention shall in no way be limited thereby. In other words, various modifications and other embodiments based on the technical spirit claimed in the claims shall be included in the present invention as a matter of course.

Example 1

Glue of 8 g was dissolved in 7 liters of deionized water, 1000 g of cuprous oxide was added and suspended therein in mixing the solution, and cuprous oxide slurry was cooled to 7° C. Cuprous oxide in the slurry was approximately 143 g/L.

Subsequently, 2000 cc of diluted sulfuric acid (concentration 24%: 9N, molar ratio (acid aqueous solution/slurry): 1.5) cooled to 7° C. was added in 1 minute. The created copper fine powder was cleaned, subject to rust prevention treatment, and thereafter dried to obtain 420 g of copper fine powder.

The reaction ended approximately 1 minute after the addition. The FE-SEM photograph of the spherical copper fine powder obtained as described above is shown in FIG. 2. As shown in FIG. 2, the average grain size of the copper fine powder was 0.09 μm. It is evident that the addition of cooled diluted sulfuric acid in 1 minute is extremely effective in attaining copper fine powder. The specific surface area BET was 6.66 m²/g. This Example 1 is a particularly favorable example even among the conditions of the other Examples.

Example 2 to Example 8

Examples of cases using, as the additive, pine resin, gelatin, carboxymethylcellulose (CMC), starch, dextrin, gum arabic, and casein are shown. In the foregoing case, the copper powder was created under all the same conditions as Example 1 other than substituting the additive. Consequently, the foregoing additives are all effective, but the addition of “glue” in Example 1 yielded the most favorable result.

Comparative Example 1 and Comparative Example 2

The copper fine powder was inspected in each case with polyethylene glycol (PEG) selected as the additive and without it. The results are shown in Comparative Examples 1 and 2. Then, the copper powder was created under the same conditions as Example 1 except the change of additive. Consequently, the additive of Comparative Example 1 yielded no effect, and in the case without additive showed inferior results as well; the grain size of the copper powder increased and copper powder having a low BET specific surface area was obtained.

The average grain size and the specific surface area of the spherical copper fine powder pertaining to the foregoing Examples and Comparative Examples were measured. The average grain size was measured based on the laser diffraction/dispersion grain size distribution measurement method, and the value of the weight cumulative grain size D₅₀ was adopted. The specific surface area was measured based on the BET method.

The results of foregoing Example 1 to Example 8 and Comparative Example 1 to Comparative Example 2 are shown in Table 1.

TABLE 1 Acid addition Reaction start Average grain BET specific Additive time temperature(° C.) size (μm) surface area (m²/g) Example 1 Glue 1 minute 7 0.09 6.66 Example 2 Pine resin Same as above Same as above 0.21 4.67 Example 3 Gelatin Same as above Same as above 0.20 4.56 Example 4 CMC Same as above Same as above 0.18 5.05 Example 5 Starch Same as above Same as above 0.19 4.95 Example 6 Dextrin Same as above Same as above 0.20 4.66 Example 7 Gum Arabic Same as above Same as above 0.25 4.00 Example 8 Casein Same as above Same as above 0.19 4.85 Comparative Example PEG Same as above Same as above 7.32 6.46 Comparative Example No additive Same as above Same as above 4.90 4.50 CMC: carboxymethylcellulose, PEG: polyethylene glycol

Example 9 to Example 12, Example 16

Next, taking typical Example 1 as the reference, results from changing the acid addition time are shown in Example 9 to Example 12. Then, the acid addition time was changed from 5 seconds to 15 minutes. Here, other than changing the acid addition time, the copper powder was created under the same conditions as Example 1. Consequently, with the acid addition time being shorter, it was possible to obtain copper powder having a small grain size and a low BET specific surface area. Since the acid addition time also affects the grain size and BET specific surface area, desirably the acid addition time is as short as possible. Though there is no need to take time in adding the acid, it is desirable to add the acid within approximately 15 minutes. The results were the same even when using the additives of pine resin, gelatin, carboxymethylcellulose (CMC), starch, dextrin, gum arabic, and casein.

Comparative Example 3 and Comparative Example 4

Next, the cases when the acid addition time was 16 minutes and 80 minutes, which are outside the conditions of the present invention, are shown in Comparative Example 3 and Comparative Example 4. Then, conditions other than changing the acid addition time and the copper powder, were the same as Example 1. In all cases, the grain size of the copper powder increased and copper powder having a low BET specific surface area was obtained, and yielded inferior results.

The results of Example 9 to Example 12 and Comparative Example 3 to Comparative Example 4 are shown in Table 2.

TABLE 2 BET specific Acid addition Average grain surface area time size (μm) (m²/g) Example 1 1 minute 0.09 6.66 Example 9 3 minutes 0.18 4.39 Example 10 2 minutes 0.15 4.94 Example 11 30 seconds 0.09 6.70 Example 12 5 seconds 0.08 6.75 Example 16 15 minutes 0.20 6.15 Comparative Example 3 16 minutes 0.40 3.80 Comparative Example 4 80 minutes 0.80 3.50 Additive: glue, Reaction start temperature: 7° C.

Example 13 to Example 17

Next, taking typical Example 1 as the reference, results from changing the reaction start temperature are shown in Example 13 to Example 17. Then, the reaction start temperature was changed from 0 to 30° C. Here, other than changing the reaction start temperature, the copper powder was created under the same conditions as Example 1.

Consequently, with the reaction start temperature being lower, it was possible to obtain copper powder having a small grain size and a large BET specific surface area. The results were the same even when using the additives of pine resin, gelatin, carboxymethylcellulose (CMC), starch, dextrin, gum arabic, and casein.

Comparative Example 5

Next, a case where the reaction start temperature was 50° C., which is outside the conditions of the present invention, is shown in Comparative Example 5. Then, other than changing the reaction start temperature, the copper powder was created under the same conditions as Example 1. In all cases, the grain size of the copper powder increased and copper powder having a low BET specific surface area was obtained, and yielded inferior results.

The results of Example 13 to Example 17 and Comparative Example 5 are shown in Table 3.

TABLE 3 Reaction start BET specific temperature Average grain surface area (° C.) size (μm) (m²/g) Example 1 7 0.09 6.66 Example 13 30 0.25 6.10 Example 14 20 0.18 6.00 Example 15 10 0.12 6.10 Comparative Example 5 50 1.20 5.76 Additive: glue, Acid addition time: 1 minute

As described above, by following the conditions of the present invention, namely, adding cuprous oxide in an aqueous medium including an additive of natural resin, polysaccharide or a derivative thereof in order to prepare a slurry containing 10 to 300 g/L of cuprous oxide, adding 5 to 50% of an acid aqueous solution at a molar ratio (predetermined number of acids/number of moles of slurry) of 1.00 to 2.00 at a time to the slurry within 3 minutes, and thereby performing disproportionation, it is possible to obtain favorable spherical copper fine powder.

Consequently, it is possible to obtain spherical copper fine powder having an average grain size of 0.25 μm or less. Moreover, with this spherical copper fine powder, it is possible to yield a specific surface area (BET) of 4.0 m²/g or more.

INDUSTRIAL APPLICABILITY

A spherical copper fine powder produced according to the present invention is very effective since the grain size of the powder is small and uniform. The powder is useful not only for oil retaining bearings and electrical brushes, but also as conductive fillers to be used as paint, paste, resin and the like. 

1-2. (canceled)
 3. A method of producing spherical copper fine powder, including the steps of preparing a slurry by adding cuprous oxide to an aqueous medium containing an additive of natural resin, polysaccharide or a derivative thereof, adding 5 to 50% of an acid aqueous solution to the slurry at a time within 15 minutes, and thereby performing disproportionation.
 4. The method of producing spherical copper fine powder according to claim 3, further including the steps of performing solid-liquid separation and water cleaning to the copper fine powder slurry obtained after the disproportionation, additionally performing alkali solution-based reduction treatment thereto, and repeating the solid-liquid separation and water cleaning of the obtained fine powder slurry to obtain copper powder.
 5. The method of producing spherical copper fine powder according to claim 4, wherein acid-based acidification treatment is performed during the course of repeating the solid-liquid separation and water cleaning of the fine powder slurry.
 6. The method of producing spherical copper fine powder according to claim 5, further including the steps of filtering the copper powder after the final water cleaning treatment, and additionally performing vacuum drying thereto in order to obtain the copper powder.
 7. The method of producing spherical copper fine powder according to claim 6, wherein the average grain size of the copper fine powder is 0.05 μm or more and 0.25 μm or less.
 8. The method of producing spherical copper fine powder according to claim 7, wherein the specific surface area (BET) of the copper fine powder is 2.5 m²/g or more and 15.0 m²/g or less.
 9. The method of producing spherical copper fine powder according to claim 8, wherein the average grain size of the copper fine powder is 0.05 μm or more and 0.21 μm or less.
 10. The method of producing spherical copper fine powder according to claim 4, further including the steps of filtering the copper powder after the final water cleaning treatment, and additionally performing vacuum drying thereto in order to obtain the copper powder.
 11. The method of producing spherical copper fine powder according to claim 4, wherein the average grain size of the copper fine powder is 0.05 μm or more and 0.25 μm or less.
 12. The method of producing spherical copper fine powder according to claim 4, wherein the specific surface area (BET) of the copper fine powder is 2.5 m²/g or more and 15.0 m²/g or less.
 13. The method of producing spherical copper fine powder according to claim 4, wherein the average grain size of the copper fine powder is 0.05 μm or more and 0.21 μm or less. 